[1] Field of the Invention
This invention relates to a process for efficiently dewatering a high moisture, porous organic solid such as brown coal, lignite, sub-bituminous coal, peat, wood chips, organic solid wastes, etc.
[2] Description of the Prior Art
High moisture, porous organic solids, especially brown coal which has an enormous reserve on the earth, are highly required to be utilized effectively. However, brown coal has the following problems:
(1) It is highly porous with many fine capillaries and its calorific value per volume is low.
(2) Water fills its capillaries and thus it is high in moisture content and low in calorific value per weight.
(3) It will be broken and become powder, when dried, due to the uneven shrinkage of its capillary structure with the result it will be hard to handle, and in danger of igniting.
Therefore, the long distance transportation of brown coal is technically as well as economically difficult, and thus its utilization is limited only at the places near the mine. Raw brown coal will not be sufficiently dewatered even if it is mechanically compressed, because the solid will soon float in the expelled water therefrom, the applied compressing load will not be transmitted to the brown coal solid structure any more and no satisfactory mechanical compression can be attained.
The following procedures are known to dewater brown coal:
(a) Drying brown coal evaporatively in advance until it has an appropriate moisture content, and then briquetting it under a mechanical compression.
(b) Dewatering brown coal mainly without evaporation by heating brown coal to a high temperature, such as 200.degree.-300.degree. C. under high enough pressure to prevent completely or partially the evaporation of the moisture therefrom,
(c) Mechanically compressing brown coal during heating under a high pressure similar to the procedure (b), as disclosed in British Pat. No. 496,680, and
(d) After heat-treating brown coal under a high pressure similar to the procedure (b), reducing the surrounding pressure to a secondary level and then mechanically compressing the coal, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 56-79189 (Austrian Patent Application 7301/79).
Procedure (a) has already been established industrially, but still has the following problems: (1) the drying consumes large latent heat for moisture evaporation, with the result of a poor economy, (2) a large mechanical compressing load is required and (3) an expensive binder often is required, depending upon the species of brown coal, to produce satisfactorily compacted briquettes. Generally, briquetting under a mechanical compression without any binder is only possible in the case of soft brown coal low in coalification rank.
Procedure (b) has advantages in improving the brown coal quality. Especially in the case of heating brown coal in saturated steam or in hot water, evaporation of moisture from the brown coal is completely suppressed, and the moisture can be removed in a liquid state from the brown coal accompanied with the corresponding shrinkage of capillaries. This procedure requires heat only to elevate the coal temperature and not to evaporate the moisture and thus the heat consumption is small. This process can dewater brown coal of lump size as it is without any breakage, because of the uniform shrinkage of the coal structure but still has the following problems: (1) The moisture remaining at the end of heating step is evaporated at the depressurizing stage to take out the coal from the high pressure to atmosphere, leaving void capillaries in the brown coal, and thus the calorific value per volume does not become satisfactorily high, and (2) a very high fluid pressure is required to leave no remaining moisture when the heating step is ended, and this is not economical. Even if the brown coal heat-treated under the high fluid pressure as above is pulverized and briquetted under a mechanical compression, no satisfactory compaction can be obtained because the quality of brown coal has been changed by the heat treatment, i.e. the soft brown coal has turned hard, the quality of brown coal has approached to that of relatively high rank coal in coalification, and the feasibility of forming biriquettes has been deteriorated.
According to the said procedure (c), brown coal can be readily deformed because it softens at a high temperature, but there are still the following problems: (1) since the capillaries shrunk by heating are filled with moisture it is hard to transmit a mechanical compressing load to the brown coal structure thoroughly and thus no satisfactory compaction can be obtained, and (2) the residual moisture at the end of heating and compaction evaporates at the depressurization, leaving void capillaries in the brown coal.
Procedure (d) has the following problems: (1) the heated brown coal is deprived of the latent heat of vaporization of moisture during the depressurization to the secondary level, and cooled and hardened, and thus it is hard to briquette the brown coal, and (2) the residual moisture evaporates at the depressurization to the atmospheric pressure after the mechanical compression, leaving void capillaries in the brown coal.